Avalanche photodiode is widely used in laser rangefinder due to high gain characteristics, but introduces highly additive noise during the time of current’s multiplication that makes laser rangefinder’s SNR meet bottleneck. This paper proposes a method of designing a high SNR’s graphene photodetector based on microcavity. The graphene’s unique optoelectronic properties make it an ideal platform for a variety of photonic applications, such as fast lasers, optical modulators, transparent electrodes, and ultrafast photodetectors. It has been recognized internationally to have dominant advantages in photodetectors due to its high carrier mobility, gapless spectrum, and frequency-independent absorption coefficient. With the wavelength of 106 μm, the mechanism of light waves’ transmission in the cavity and the graphehne’s absorption are studied by using optical transmission matrix method and scattering matrix method; the light absorption model of the graphene photodetector based on microcavity is established. Device’s final quantum efficiency reaches 912%, respectively reaches 0778 A·W-1, its full width at half maximum (FWHM) reaches 6 nm; the influence between graphene’s position in the microcavity and device’s absorption shows that device’s absorption’s peak value changes periodically with graphene’s position under resonant condition, and the variety of length of microcavity does not have any influence on the peak value, but changes the graphene’s position when absorption reaches peak value, on the condition that the length of microcavity is n times of half of wavelength, the number of device’s absorption peak value is 2n with the variety of graphene’s position, and all the peak values are symmetrical with respect to the center of microcavity, the final graphene’s position is 0402 8 mm away from the top mirror of microcavity, and the absorption reaches 94%, Compared with single layer graphene, the absorption rate increases 16 dB; By solving SNR equation of the graphene photodetector based on microcavity and SNR equation of the avalanche photodiode, eventually finds that the SNR of the graphene photodetector based on microcavity is 903, which raises 10 dB compared with the avalanche photodiode’s. Theoretical analysis shows the graphene photodetector based on microcavity has high absorption rate, high quantum efficiency, and high SNR. In this paper, the research achievements provide a theoretical reference to update and design higher SNR photodetector used in laser rangefinder’s receiving system.